Stiffness and bending strength tester for thin sheet materials



Feb. 22, 1949. R. D. DE WAARD ETAL 2,462,826

STIFFNESS AND BENDING STRENGTH TESTER FOR Il-IIN SHEET MATERIALS Filed July 51, 1947 4 Sheets-Sheet l l D|V.= .ol RADIAN K #WEY 2 Filed July 31, 1947 Feb. 22, 1949.

' R. D DE WAARD ET AL STIFFNESS AND BENDING STRENGTH TESTER FOR THIN SHEET MATERIALS 4 Sheets-Sheet 2 Feb. 22, 1949.

R. D. DE WAARD ETAL STIFFNESS AND BENDING STRENGTH TESTER Filed July 31, 1947 FOR THIN SHEET MATERIALS 4 Sheets*Sheet, 3

"III I 36 n9 5 INVENTOR S QW44P0 ATTORNEY D. DE WAARD EI'AL STIFFNESS AND BENDING STRENGTH TESTER Feb. 22, 1949.

FOR TH IN SHEET MATERIALS 4 Sheets-Sheet 4 Filed July 51, 1947 Patented Feb. 22, 1949 STIFFNESS AND BENDING STRENGTH TESTER FOR THIN SHEET MATERIALS Russell Dick De Waard, Riverside, Charles Rule Stryker, Cos Cob, and Albert Franklin Blockman, Springdale, Conn., assignors to American Cyanamid Company, New York, N. Y a corporation of Maine Application July 31, 1947, Serial No. 765,166

3 Claims. 1

This invention relates to an apparatus for testing the stiffness of thin sheet materials. More particularly, the invention is directed to an improved apparatus for measuring the strength, maximum deformation and stiffness of thin sheet materials in bending.

The device of the present invention involves the testing and measuring of these properties in thin sheet materials such as paper, cardboard, fibrous materials, synthetic resins, plastics, hard rubber, composition board, leathers, imitation leathers, metal foil, spring wire and strip, textiles, thin flexible sheet metals and other similar materials, by bending a weighted strip of the material through the angle required to produce failure or maximum deformation and measurement of the force applied. This provides an index of the strength, maximum deformation, and stiffness of the material.

Numerous treatments of thin sheet materials with impregnating agents and coatings, such as sizes and resins, waterproofing agents and the like, and chemical baths, as Well as modes of preparation, have been developed to increase the value of such materials for specific purposes by enhancing certain properties or imparting new properties thereto. Such treatments often affect the mechanical strength, maximum deformation and stiiiness characteristics of these materials in bending either by enhancing these properties or weakening them. In some instances these treatments augment the characteristic known an brittleness. At times such characteristic is eliminated. In general any material which cracks or experiences a sharp definite failure at relatively small strains (deformation) has a brittle character. The degree of such property exhibited depends upon variables such as the type of paper, textile, or the like, its mode of preparation, the extent of impregnation, characteristics of the treating agent, for example, the degree of resin cure, if resin treated, and other variable conditions. Brittleness, as such, can only be qualitatively expressed, and therefore in the absence of quantitative expression, no definite standards for meeting specifications required can be established. However, the unit strength, maximum deformation, and stiffness characteristics are susceptible of quantitative measurement, and these properties may be evaluated to establish quantitative standards for thin sheet materials.

In order to eliminate undesirable properties, such as cracking or brittleness of such materials,

the extent to which the different variables introduced by methods of preparation and treatment of these thin sheet materials affect the properties in question must be determined. An evaluation of the relative strength, maximum deformation and stifiness of the materials affords a basis for such determination. Such tests, when conducted under varying conditions with respect to methods of preparation; amounts and nature of the coating, or impregnating agents, and/or treating baths, will by the results thereof directly suggest how to improve the characteristics desired and eliminate the undesirable properties, such as strength, stiffness, flexibility and brittleness.

The improved apparatus of the present invention is eminently applicable in the research and development conducted with a view to improving and standardizing these mechanical properties of thin sheet materials.

In the past several devices have been produced for testing the stiffness and bending strength of sheet materials. However, such devices contained several serious disadvantages, the principal one being their inability to obtain good precision in the measurement of these properties in testing very thin sheets, such as paper, wherein a very low range of stresses is necessarily employed. In addition, these devices for the most part have a serious limitation in that evaluating strength and stiffness of materials by measurements with such apparatus, the results obtained from samples of different thickness are not directly comparable.

Also, many of the prior art devices embody apparatus whereby a free edge of the sample strip being tested plays a critical part in the test, necessitating a difficultly obtainable smooth edge as contrasted With a feathered edge, in order to obtain results even approaching the requisite precision. Such apparatus suffers a further disadvantage in that the sample strip is mounted vertically with the lower end attached to a support. The upper end being free, it is necessary to take into account the weight of the strip itself to compensate for the initial bending caused by this weight.

In still other devices the holding means for the sample strip does not permit an adjustment in order to maintain the lengthwise center of the sample coincident with the axis of rotation when the sample thickness varies. As a result, these devices are not operative to test samples of varying thickness, or at best if the device does accommodate samples of different thicknesses, the re- 3 sults of tests on the same cannot be directly compared. Furthermore, this serious limitation causes the introduction of diiferent types of bending stresses in samples of different thicknesses.

In addition, none of the prior art devices are equipped with any means which permits direct computation of the bending force solely from the moment produced by the pendulum arm or other means exerting the force when testing sheets of diiferent thicknesses. In these devices static balance of the operating members does not exist for samples of different thicknesses, and therefore a compensating factor must additionally be calculated in order to determine the true magnitude of the force actually applied.

The present invention comprehends an improved apparatus for measuring the strength, maximum deformation and stiffness of thin sheet materials by providing a pair of relatively movable members for bending a strip of the material to be tested hereinafter called the test strip, which members are provided with advantageous novel features. The pair of members are independently mounted for relative rotation about a,

common axis, each member having holding or clamping means for opposite ends of the segment of the test strip which is subjected to bending. The bending stress is applied upon positive rotation of one of said members, by the relative rotation of the second member which is set up through resistance of the strip to bending. This resistance is measured in terms of the progressively increasing force exerted on the strip by continued relative rotation of the second member.

Thus, it is an advantage of the invention that the segment of the test strip subjected to bending has both ends thereof secured whereby difliculties encountered with testing devices wherein one edge of the test strip is free are obviated.

According to the invention the test strip holding or clamping means of the relatively rotatable members are provided with calibrated adjustable means for varying the span, that is, the distance longitudinally on the test strip between the holding means, in substantially fixed relation to the thickness of the test strip.

This accurately calibrated adjustable span feature is a distinct advantage of the invention in that it not only permits the testing of strips of varying thicknesses, but in addition makes it possible to make a direct comparison of results of tests on strips of different thickness in terms of the properties measured, that is, strength, maximum deformation and stiffness. In addition, the adjustable means for the test strip holding means permits setting the holding means on the test strip equidistance from the axis of rotation regardless of test strip thickness. In other words, the device provides for maintaining the lengthwise center of the test strip coincident with the axis of rotation for samples of varying thickness. As stated above, there is a' fixed ratio between thickness of the test strip and its length. This factor, combined with the adjustable feature of the test strip holding means insures that the same kinds of bending stresses are present in all samples regardless of thickness during operation of the device.

One of the principal features of the invention is the provision of a calibrated adjustable counterbalance mechanism for the relatively rotatable member which actually applies the bending stress to the test piece, as distinguished from the positively rotated member. This force applying member mounted independently for rotation about a common axis with the positively rotated member comprises a calibrated adjustable test strip holding means for setting the span and a moment arm, such as an adjustable pendulum, adapted to carry variable weights or an equivalent adjustable force exerting device. The calibrated adjustable counterbalance for this member accommodates any adjustment of the span, whereby regardless of span the member is statically balanced. Due to the calibrated counterbalance the moment about the axis of rotation is readily determined solely from the known weight attached to the moment arm and the length of the latter. The variation in span causes displacement of the holding means from or toward the axis of rotation which in turn increases or decreases the total moment about the axis. Error due to this factor is completely eliminated by compensating for the change in moment through correct adjustment of the calibrated counterbalance. V

The important advantages flowing from this feature are that measurements can now be made with high precision even in the low range of stresses necessary when testing thin sheets such as paper. In addition, the moment about the axis for any weight added to the moment arm may readily be determined for any adjustment of span, and results on test pieces of different thickness are directly computed and comparable without computing the compensating factor for span variation.

The positively rotatable member may be rotated by any suitable means, such as a manual rotation by means of a crank connected to the supporting shaft of the member or through suitable gearing thereto. Alternatively, this holding member may be positively driven by mechanical or electrical means such as motors through a belt drive or a earing and shaft mechanism. The automatic drive is much more preferable than a manual drive, since the positively rotated member may be driven at a predetermined angular velocity with a resulting smooth application of load on the test strip, thus eliminating any tendency toward the more or less spasmotic application of load by manual means.

The apparatus further embodies means for measuring the angles through which each rotatable member is displaced. A fixed scale or scales may be employed in conjunction with pointers actuated by movement of the rotatable members, or fixed pointers mounted on the frame may be used in conjuction with movable scales carried by or actuated upon angular displacement of the relatively rotatable members.

The device of the present invention is more fully described and illustrated with reference to the accompanying drawings, which show a preferred embodiment, and are not intended to constitute a limitation of the invention, in which:

Fig. 1 is a front elevation of the paper stiffness tester showing the relatively rotatable members and the test strip holding means with the novel span adjusting feature;

Fig. 2 is a side elevation of the'device as assembled with relation to the housing;

Fig. 3 is a Vertical section taken on line 33 of Fig. 1, being partly in side elevation and showing the novel counterbalance mechanism for the relatively rotatable force measuring member;

Fig. 4 isa plan View with the housing removed and parts broken away;- 7 i T Fig. 5 is. a perspective of the novel span adjusting means and the test strip clamping means;

Fig. 6 is a diagrammatic representation of the mechanics of the device;

Figs. 7 and 8 are enlarged sectional views of the test strip and clamping jows of the test strip holding means in initial position and position during test, respectively, and

Fig. 9 is an enlarged view showing the relation between test strip thickness and the span between the clamping jaws of the holding means.

Referring first to Figure 6, which is a schematic drawing showing the operating principles and mechanics of the device, the tester and its operation will be first more or less generally described.

A represents the assembly of the positively rotated member and test strip holding means mounted to rotate about the axis 0. This assembly A is preferably motor-driven at a predetermined angular velocity. B represents the assembly of the second dependently rotatable member and test strip holding means which also is freely and independently rotatable about the axis 0. The ends of the test strip designated as the Sample are fixedly mounted between the two rotatable members by means of the respective holding means. Assembly B carries rigidly secured thereto or integral therewith, an assembly 0 comprising an extended arm to which weights W are adjustably and removably secured, thus providing a moment arm affording an adjustment of the range of bending moments applied to the test strip.

The span of longitudinal distance on the test strip between the holding means of assemblies A and B is variable by means of the calibrated adjusting means therefor. By settin each of said holding means equidistant from the axis 0', the center of the test strip coincides with the axis of rotation. In addition, the adjustable span device accommodates test strips of different thickness.

The assemblies B and C are provided with an adjustable counterbalancing mechanism so that for any span adjustment, the assembly is always in static balance with the result that the bending moment or force applied to the strip consists solely of. the moment exerted by weight W acting through the distance L on the arm of assembly C from the axis 0. Thus, for any given span of the holding means of assemblies A and B on the test strip, the moment about axis 0 of any weight W added to C can be readily determined. That is, Moment=WL sin 4 Where =angle of displacement of the moment arm of assembly C from the vertical.

During a test, the angular displacements of the positively rotated assembly A and of the dependently rotated assembly B are measured onsuitable means, such as an arcuate scale, in terms of angles 0 and respectively. The actual angle through which the test strip is bent is a, which is simply the difference between the angles 0 and eh, i. 8. oc=(0-) The three properties, namely, unit strength, maximum deformation and stiffness are evaluated by calculating the same from the following formulae using readings obtained during tests of the angles through which the relatively rotatable members moved, the magnitude and position of the weight on the moment arm, and the'dimensionsof the sample.

6 Unit'strength Force IcWL sin :1: Unit area bd Unit defamation Change in length kda Unit length m 1 Unit stifiness Unit strength lo WlL sin t Unit deformation bd a Definition of terms in Equations 1, 2 and 3 k, k, and k" are proportionality constants is the angular displacement of the momentarm of assembly C from the vertical 0: is the angle of bend of the test strip and equals (tit) as read on the instrument scale 0 is the. angular displacementof positively rotated assembly A W is the weight added to the moment arm L is the distance from the center of weight W to the axis of rotation 0 Z is the length of the test strip and is equal to the span or the distance longitudinally on the test strip between the holding means b is the width of the sample dis the thickness of the sample.

The above equations were derived by the use of several assumptions namely that the materials tested are isotropic and elastic and are subjected to pure bending. None of these conditions are perfectly satisfied in practice, but the equations are rendered quite valid in evaluating the properties in question by restricting their scope. Thus, in performing tests the ratio. of test strip length (l) to thickness (d) must be maintained substan- Equation I Equation 2 Equation 3 tially fixed. This is made possible by the novel span adjusting means of assemblies A and B.

It should be noted that the angle of bend (a) of the test strip is always of a relatively small value where the characteristic of stiffness is being computed, that is, where they data obtained from the test is to be used in Equation 3. In such case the test strips do not fail mechanically in evaluating stiffness and (ti-qt) remains small. On the other hand, the angle of bend (on) of the test strip as represented in Equation 2 is always equal to the difference between (0), the angular displacement of the positively rotated assembly A and the angular displacement of the moment arm of assemblies B and C at the instant the test strip fails mechanically during the test. Thus, in computing unit deformation at has a relatively large value.

The evaluation of the properties computed from these test data thereby affords a quantitative measure, at least relatively of highly important characteristics of thin sheet materials and provides a basis for improvement in such mechanical characteristics by modification of methods of preparation and treatment.

Referring first to Figure 2 for specific details of construction of the preferred embodiment, a metallic housing I for the device is provided with a face plate 2-, the latter carrying the external opera-ting parts of the apparatus, which are shown more advantageously in Figure 1.

Figure 1, a front elevation of the paper stiffness tester shows face plate 2 through which the upper assembly 3 of thepositively driven relatively rotatable member and the lower assembly 4 of the second relatively rotatable member with calibrated adjustable holding means for' the test strip are mounted independently for rotation about the axis and in operative relation to the balance of the device situated within the housing I. The novel test strip holding means of assemblies 3 and 4 will be more fully described hereinafter with reference to Figure 5.

Face plate 2 carries an arcuate scale 5 calibrated in radians by which angular displacement of assemblies 3 and 4 during test are indicated.

A verticall disposed moment'arm, 6 is fixedly attached to and dependent from lower assembly 4 and is adapted to carry a weight I, the position of which an arm 6 is adjustable to provide avariation in the range of applied moment during operation of the tester.

The lower assembly 4 actuates a pointer 8 upon angular displacement thereof. The pointer 8 serves to indicate in conjunction with scale 5, the angle or the angular displacement of the moment arm 6 and weight 1 from the vertical.

A second pointer 9 is attached to assembly 3 of the positively driven member and rotates therewith to indicate angle 0. The difference between the two readings (Ii-g5) gives the true angle of bend in the test strip, above referred to as oz.

Referring now particularly to Fig. 3 of the drawings, a hollow cylindrical sleeve I0 is mounted for rotation within a cylindrical bearing member H, the latter being supported by insertion of one end thereof through an aperture in face plate 2, and having flanges by which it is rigidly secured to the inside of the plate 2. The sleeve I0 is provided at the front end thereof with a rectilinear plate extension l2 forming that part of the positively driven rotatable assembly 3, which carries one of the test strip holding means. A shaft I3 is fitted within the sleeve It], the former vbeing independently mounted with respect to the sleeve Ill by means of ball bearings i4 and I5 located between the sleeve and the shaft at both ends of the shaft. The shaft I3 is provided at the front end thereof with an integral dependent plate member 16, which forms that part of the lower relatively rotatable assembly 4 carrying the second test strip holding means.

At the opposite end of the sleeve i 3 is provided an annular hub I l which is rigidly secured to the sleeve by means of set screw l3. The hub IT has an arm 33 dependent therefrom, provided with a weight 26, which serves as a counterbalance for the upper rotatable assembly 3 comprising the span adjusting and test strip clamping jaw attached to plate 12. The weight 20 is provided with a threaded bolt 2i and a retaining member 22, so that additional Weight may be added or a portion thereof removed in order that positively driven upper assembly 3 be maintained in approximately static balance for any given span adjustment.

The cylindrical sleeve :3 and the shaft l3 are independently free to oscillate on the ball bears l4 and I5.

The lower rotatable assembly 4 comprising the plate l6 carryin the span adjusting and test strip holding means is provided with a novel calibrated adjustable counterbalance constructed as follows. The shaft i3 extends beyond the end of sleeve ll! which carries the hub l7. On this extension of the shaft an annular hub 23 is fixedly attached by means of the set screw 24. The hub 23 carries an arm 25 to which is attached the adjustable counterbalance for the lower assembly 4.

This adjustable counterbalance comprises a 8 weight 26 provided with beveled'fianges 21 slidably mounted between two beveled strips 28 attached to the arm 25, which strips have a scale calibrated thereon in terms of the distance of weight 26. from the center of shaft'l3. The

flanged weight 25 is provided with a Vernier scale registering with the scale on strips 28. The weight 26 also carries threaded rod 29, which has attached to its upper end a micrometer dial 30 in threaded engagement therewith. The dial 30 is supported by a bracket 3! extendin from arm 25 and integral therewith, through which the portion of rod 29 carrying dial 3!] extends. The Vernier scale together with the micrometer dial 33 provides means for accomplishing an accurate adjustment of the counterbalance for the variable span of the lower span adjusting and clamping jaw assembly 4. Thus, the dial 30 may be turned to raise or lower the weight 26 on the arm 25, which in turn increases or decreases-the moment produced by the weight 26 with respect to the axis of rotation O, which is coincident with the center of shaft i3. Thus, for any variation in span of the test strip holdingmeans the lower rotatable assembly 4, which alone measures the force applied to the test strip, is at all times in static balance. The counterbalance compensates for any adjustment in span required to accommodate test strips of different thickness. If desired, weight 26 may be provided with a threaded bolt and retaining member to provide means for adding weights thereto as in the cast of bolt 2| and retaining member 22 attached to weight 20 for the positively rotated assembly 3.

The positive drive mechanism for the upper positively rotated assembly 3 may also be best described with reference to Figure 3 and, in addition, Figure 4. a

An annular sleeve 32 is mounted externally on the bearing II and is provided with teeth forming a sprocket 33. An annular clutch member 34 is also positioned externally on bearing I l between the sleeve 32 and the sub 17. The clutch member 34 is movably keyed to the hub H by means of a plurality of pins 35 attached to the member 34 and. slidably mounted in holes provided in the periphery of the hub E1. Normally'the clutch member 34 is held in contact with the huh I! and is separated from the sleeve 32 by means of a coil spring 36 which has one end seated in an annular recess of sleeve .32 and the opposite end bearing against member 34. The adjacent ends of sleeve 32 and clutch 34 are provided with interlocking teeth 31 and 38 to provide a positive locking drive as distinguished from the frictional drive.

A clutch operating lever 39 is provided as shown in Figures 1 and 4. The lever is pivoted at 40, also shown in Fig. 1, and is bifurcated at its operating extremity. The bifurcated arms M and 42 of the clutch operating lever 39 are'keyed in an annular groove 43 in the clutch member 34, as shown in Figure 4, as well as Figure 3; I

The sprocket 33 on sleeve 32 is adapted to receive a chain 44 driven by sprocket 45 attached to the shaft of a motor 46. Thus, upon operation of the clutch lever 39 the teeth 3'! and 38 are interlocked by movement of the clutch member 34 into an engagement with the sleeve 32. With the.

motor 46 runningupon operation of clutch lever 39 to engage the clutch member 34, the chain and,

sprocket drive causes rotation of sleeve 32, the: clutch member 34, the hub l1, and consequently; the sleeve l0, so that the upper rotatable assembly.

3 is positively driven at a predetermined adjustable angular velocity.

It is to be noted that the clutch lever 39 is shown as manually operated. However, a self-releasing clutch drive may be employed for driving the assembly 3. Suitable mechanism for this automatic feature is shown in U. S. Patent 2,314,193 to Boor and Bourdelais dated March 16, 1943. In adopting this automatic self-releasing clutch drive, no alteration in construction of the device is necessary except that the sprocket and chain drive for sleeve 10 as shown is eliminated, and a drive shaft with a bevel gear or the like coacts with another bevel gear substituted on sleeve It] for sprocket 33.

The angular displacement of the upper assembly 3 is indicated on scale by means of a pointer 9 comprising a flat band attached to rotatable plate extension l2 of sleeve Ill. The pointer 9 carries a curved extension 50 at the upper extremity thereof to afford accurate alignment with the zero mark on scale 5.

A second pointer 8 also comprising a flat band having an angularly bent upper extremity 5| adapted to freely ride in a semi-circular groove 52 in face plate 2 is rotatably mounted on a forward extension 53 of cylindrical bearing member II. A retaining ring 54 is mounted on extension 53 to secure the hub of pointer 8, The pointer 8 carries at its lower extremity, which extends below the shaft l3 and the extension 53 of bearing member II, a forwardly extending flange 55 presenting a contact surface.

The lower span adjusting and clamping jaw assembly 4 carries a moment arm 6 rigidly secured to plate 16 and vertically dependent therefrom. The arm 6 is adapted to carry a weight I adjustably mounted thereon by means of set screw 56. A scale 51 is provided on the arm 6 calibrated in units of length representing distance of adjustably mounted weight I from the axis of rotation, namely, the center of shaft I3, for direct measurement of moment thereby facilitating computation of the consequent force on the test strip when the testing device is in operation.

A pin 58 extends horizontally from the lower plate I6 into actuating relation with the flange 55 of the pointer 8. Thus, upon positive drive of the upper assembly 3 with the test piece fixed between the upper and lower relatively rotatable assemblies 3 and 4, movement of the upper assembly 3 will induce bending of the strip, which, in turn, in resisting the bending will cause a consequent relative angular displacement of lower assembly 4 and the moment arm 6. This in turn will cause pin 58 to actuate pointer 8 to indicate the angular displacement of thelower assembly 4 and moment arm 6 during operation of the tester.

The novel calibrated span adjusting device for the test strip holding means and the construction of the test strip holding means of the upper and lower rotatable assemblies 3 and 4 are shown in detail in Figure 5 and also Figure 1.

The positively rotated upper assembly 3 comprises the rectilinear plate extension l2 of cylindrical sleeve H3. Attached to the outer face of plate l2 at the sides thereof are vertically disposed beveled strips 60 and 6|. Slidably mounted between strips 60 and BI is a beveled plate 62 which carries integral therewith a centrally disposed lug 63 projecting outwardly at right angles to the plate. The lug 63 is provided with a pair of holes located at the inner and outer portions of its lower edge. These holes receive studs 54 and 65 (shown in Figure 4) of a modified U-clamp provided for clamping the test strip in a fixed position. The clamp comprises studs 64 and 65 fixedly secured at their corresponding ends to a beveled clamping arm 66 and provided at the other ends thereof with a yoke 5? fixed thereto. Beveled clamping arm 66 and lug 63 form the clamping jaw of the test strip holding means. Yoke 61 has a centrally positioned threaded hole therein which receives a knurled headed clamping screw 58 the opposite end of which abuts against the lug 53 between the stud holes. One end of the test strip is inserted between arm 66 of the clamp and the lug 63 so that upon threading of the screw 68 the arm 66 is moved against the test strip toward the side of lug 63 by movement of yoke 61 on the screw 68. This securely clamps one end of the test strip to the upper positively rotated assembly 3. The clamping surface of lug 63 lies in a plane common to the axis of rotation.

The lower independently rotatable assembly 4 comprises the vertically dependent plate member :5 integral with the front hub portion of shaft i3. Plate iii in turn carries moment arm 6 to which weight '1 is removably secured. The dependent plate member 5 has attached to its outer face at the sides thereof vertically disposed beveled strips 85 and B I. A beveled plate 82 is slidably mounted between strips 85 and Si and carries integral with it a centrally located lug 53 projecting outwardly from plate 82 and at right angles thereto. A pair of spaced holes are provided in lug 83 at the inner and outer corners of the top portion. These holes receive a pair of studs 85 and 35, the common ends of the latter being connected by a beveled clamping arm 85 and yoke 3? forming with lug 83 a modified U-clarnp. Beveled arm 85 and lug form the clamping jaw of the test strip holding means for lower assembly 5. The clamping SUI- face of lug 83 lies in a plane common to the axis of rotation, and also common to the plane of the clamping surface of lug 63 when the apparatus is in initial test position. Yoke 8? is provided with a centrally positioned screw threaded hole which receives a clamping screw 88, the opposite end of which abuts against the side of the lug 83 between the stud holes.

The clamping action is identical to that of the test strip clamp for the upper positively rotated assembly 3, so that the two free ends of the test strip are secured to the upper and lower relatively rotatable assemblies 3 and l, respectively.

Lugs 63 and 83 have attached thereto in the upper and lower edges, respectively, threaded bolts 63 and 89. Plates I2 and it; are provided at the upper and lower extremities, respectively, with forwardly projecting apertured brackets 15 and 9D. The threaded bolts 69 and 89 are provided with micrometer dials H and 9! in threaded engagement therewith abutting brackets H3 and 9! Beveled strips and 6|, 8!) and 8! have scales 12 and 82 marked thereon which are calibrated in units of length for measuring the distance of the clamping arms 66 and 86 and the coacting lugs 63 and 83 from the axis of rotation. Sliding plates 62 and 82 have Vernier scales I3 and 93 calibrated thereon for making an accurate adjustment of the span or distance longitudinally on the test strip between the pair of clamping jaws 53 and S683. In addition the scales provide the means for setting the clamping jaws equidistant from the axis of rotation whereby the center of the test strip will always coincide with the axis regardless of thickness of the strip. Adjustment of the test strip clamping jaws is made merely by turning the micrometer dials H and 9! thereby moving the slidin plates 62 and 82 which carry lugs 63 and 83 and clamping arms 65 and S5. The test strip is then inserted between these elements and clamp screws 68 and 88 are tightened to fix the strip in test position.

In accordance with the invention, the calibrated span'adjusting device permits accommodation of test strips of varying thicknesses. Referring to Figure 9, the fixed relation of adjustable span to test strip thickness is clearly shown. For a given thickness 0. of the test strip, the span will be adjusted to a value of a. With a strip of thickness b, greater than a, the span is adjusted to a value b, so that a substantially fixed ratio is maintained between thickness and span, or in other words it (nearly so) It is important to note that both elements, namely clamping arm 58 and lug 63 or arm 86 and lug 83 are jointly moved an equal distance to or from the axis of rotation by adjusting the span so that both sides of the test strip are clamped at equal distances from the axis, or in other words a line drawn through the strip connecting the edge of the beveled clamping arm 66 and the clamping edge of lug 53 will be horizontal. This construction insures the maintenance of a fixed or substantially fixed ratio between the test strip length l and test strip thickness d, and also coincidence of the axis of rotation and the lengthwise center of the test strip. Length 1 is actually equal to the span which in Figure 9 is either a or b depending on the value of thickness d which is a or b, respectively. This permits direct comparison of results obtained by measurements using samples of different thickness.

A test utilizing the device of the present invention is conducted in the following manner. A test strip, for example, resin treated paper, is selected for test. The qualitative thickness is examined and a proper weight 1 is empirically chosen and is secured to moment arm 6 at a selected distance on scale 56. The thickness d of the test strip is measured and the span is adjusted by means of the calibrated adjusting device of assemblies 3 and 4. In setting the span and upper and lower clamping jaws are maintained equidistant from the axis of rotation or center of shaft l3. The position of the lower clamping jaws is noted from the Vernier scales 9293 and weight 26 of the calibrated adjustable counterbalance is adjusted by means of micrometer dial 3!! to give the same reading on the Vernier scale carried by weight 26 and beveled strips 28. Thus, the test strip holding means of the lower rotatable assembly 4 is positioned exactly the same distance from the axis of rotation as the weight 26 of the adjustable counterbalance. This maintains the lower rotatable assembly 4 is static balance with the exception only of weight I on moment arm 6. Therefore, the

moment due to weight 1 located at a fixed position on arm 5 will be the sole factor in determining the force exerted on the test strip upon angular displacement of lower assembly 4 from the vertical during the test.

The test strip is inserted between clamping arm 66 and lug 63 and clamping arm 86 and lug 83, and fixed in test position by tightening screws 68 and 88. With the span or length of the strip to be subjected to bending set in fixed position, the

apparatus is ready for operation. Pointers 8 and 9 coincide with the zero reading on scale 5. Motor 46 is started by means of switch 94 (shown on face plate 2 in Fig. 1) and clutch lever 39 is depressed causing clutch member 34 to move axially on the bearing member I lwhereby member 34 is lockedfor rotation by engagement of teeth 38 with teeth 3'! on sleeve 32 carrying sprocket 33 and belt 44 for positive rotation of bearing member It by means of motor 46. With the clutch engaged, pins 35 on clutch mem; ber 34 seated in recesses in hub l1 cause rotation of cylindrical sleeve Ill to which hub I1 is keyed. Thus, the upper rotatable assembly 3 is positively driven at a predetermined angular velocity. Rotation of assembly 3 to which the upper end of the test strip is secured. by clamping arm 66 and lug 63 causes relative rotation of the lower rotatable assembly 4 due to resistance of the strip to bending. Fig. '7 shows the relative positions of the test strip clamping jaws of assemblies 3 and 4 before rotation of the upper assembly 3 is initiated. Figure 8 shows the relative positions and angular displacements of the upper and lower clamping jaws after operation of the positively driven upper assembly 3 has been initiated.

Rotation of lower assembly 4 causes angular displacement of pointer 8 by engagement of pin 58'carried by plate It with flange 55 of pointer 8. The weight distribution of pointer 8 withrespect to the axis of rotation is such that it will remain in its attained position of angular displacement from the vertical when pin 58 no longer causes rotation. Pointer 9 is carried by upper assembly 3 and rotates therewith.

When the test strip is stressed by continued bending to the point of failure by cracking or to a predetermined point of maximum deformation, the operator releases clutch lever 39 and reads the position of pointer 9 on scale 5 immediately. Lower assembly 4 drops to its original vertical position due to elimination of the resistance of the paper, and since the cracked test strip is :also still attached to the upper assembly 3, the latter also swings back toward its initially vertical position. I V V Pointer 8. remains in the angular position attained at the time the test strip failed at which time pin 58 of lower assembly 4 drops away from driving engagement with the pointer 8. A read ing is then taken of the position of pointer 8 on scale 5. The moment exerted by weight I on arm 6 is directly and readily computed from known weight W, the known distance L (indicated on scale 51) of the weight W from the axis of rotation (center of shaft l3) and the measured angle of displacement indicated by pointer 8 on scale 5 when the test strip failed. Thus, the moment=WL sin P v The properties in question namely unit strength, unit deformation and unit stiffness are computed from this data plus the measured angle 0, the angle of displacement indicated by pointer 9 using the Equations 1, 2 and 3 given above.

The device of the present invention is simple in operation and is constructed to be portable. The invention provides a device for evaluating stiifness quantitatively with a high precision in the low range of stress employed in testing thin sheets, for example, paper, which precision was hitherto unattainable. The values of unit strength, unit deformation and unit stiffness may be readily calculated from merely the measurement of two angles and one moment.

In the claims appended hereto, clamping arms 66 and 86 together with lugs 53 and 83 will be referred to as test strip holding means and pointers 8 and together with scale 5 as indicating means.

Various changes in construction or arrangement of the device may be made without altering the scope of our invention, the extent of which is determined. by the following claims which are to be broadly construed.

What we claim is:

1. An apparatus for testing the stiiiness of thin sheet materials comprising a pair of rotatable members mounted independently for relative rotation about a common axis, one of said members comprising a hollow cylindrical sleeve having a radially disposed plate extension at one end thereof, the other of said members comprising a shaft journaled within said sleeve and having a plate extension at the corresponding end thereof oppositely radially disposed to the plate extension of the sleeve, calibrated adjustable means carried by each one of said plate extensions for attaching a test strip of material to both of the plate extensions of the relatively rotatable members, comprising plates slidably and adjustably mounted on said plate extensions, said plates carrying test strip clamping devices comprising fixed clamping surfaces located in a plane common to the axis of rotation of said members and clamping members operable to secure the test strip against the clamping surfaces, said calibrated adjustable means being operable to set said clamping devices on each plate extension equidistant from the axis of rotation on said strip and adjustable to vary the distance of said clamping devices from said axis, driving means for positively rotating said sleeve and plate extension at a predetermined angular velocity whereby said shaft and plate extension is relatively rotated through resistance of the test strip to bending, comprising an electric motor, a belt and sprocket assembly in actuating relation between said motor and said sleeve, and a lever operated clutch means interposed between said belt and sprocket assembly and said sleeve extension for releasably engaging said sleeve in driven position, an adjustably Weighted pendulum arm carried by the plate extension of said shaft for exerting a progressively increasing force 010- posing resistance of said test strip to bending upon continued relative rotation of said shaft and plate extension from a normal position of rest, a calibrated adjustable counterbalancing device for said shaft and plate extension mounted on the shaft so that said shaft and plate extension is maintained in static counterbalance regardless of distance of the clamping device from said axis of rotation whereby the weighted pendulum arm is the sole force exerting factor, a pointer carried by said plate extension of the sleeve, another pointer independently mounted for rotation about the axis and actuated upon rotation of said shaft and plate extension, and a scale for indicating in conjunction with the pointers the angular displacement of the relatively rotatable members.

2. An apparatus for testing the stifiness of thin sheet materials comprising a pair of rotatable members mounted independently for relative rotation about a common axis, one of said members comprising a hollow cylindrical sleeve having a radially disposed plate extension at one end thereof, the other of said members comprising a shaft journaled within said sleeve and having a plate extension at the corresponding end thereof oppositely radially disposed to the plate extension of the sleeve, calibrated adjustable means carried. by each one of said plate extensions for attaching a test strip of material to both of the plate extensions of the relatively rotatable members, comprising plates slidably and adjustably mounted on said plate extensions, said plates carrying test strip clamping devices, said calibrated adjustable means being operable to set said clamping devices on each plate extension equidistant from the axis of rotation on said strip and adjustable to vary the distance of said clamping devices from said axis, driving means for positively rotating said sleeve and plate extension at a predetermined angular velocity whereby said shaft, and plate extension is relatively rotated through resistance of the test strip to bending an adjustably weighted pendulum arm carried by the plate extension of said shaft for exertin a progressively increasing force opposing resistance of said test strip to bending upon continued relative rotation of said shaft and plate extension from a normal position of rest, a calibrated adjustable counterbalancing device for said shaft and plate extension mounted on the shaft so that said shaft and plate extension is maintained in static counterbalance regardless of distance of the clamping device from said axis of rotation whereby the weighted pendulum arm is the sole force exerting factor, a pointer carried by said plate extension of the sleeve, another pointer independently mounted for rotation about the axis and actuated upon rotation of said shaft and plate extension, and a scale for indicating in conjunction with the pointers the angular displacement of the relatively rotatable members.

3. An apparatus for testing the stiffness of thin sheet materials comprising a pair of rotatable members mounted independently for relative ro tation about a common axis, one of said members comprising a hollow cylindrical sleeve having a radially disposed plate extension at one end thereof, the other said members comprising a shaft journaled within said sleeve and having a plate extension at the corresponding end thereof oppositely radially disposed to the plate extension of the sleeve, calibrated adjustable means carried by each one of said plate extensions for attaching a test strip of material to both of the plate extensions of the relatively rotatable members, comprising plates slidably and adjustably mounted on said plate extensions, said plates carrying test strip clamping devices comprising fixed clamping surfaces located in a plane common to the axis of rotation of said members and clamping members operable to secure the test strip against the clamping surfaces, said calibrated adjustable means being operable to set said clamping devices on each plate extension equidistant from the axis of rotation on said strip and adjustable to vary the distance of said clamping devices from said axis, driving means for positively rotating said sleeve and plate extension at a predetermined angular velocity whereby said shaft and plate extension is relatively rotated through resistance of the test strip to bending,

comprisin an electric motor, a belt and sprocket 16 9 REFERENCES CITED The following references are of record in the file of this patent:

Number Number UNITED STATES PATENTS Name Date Wille June 12, 1923 Schopper Oct. 27, 1925 Midgley July 24, 1928 Schopper July 9, 1935 Tour et a1 Ju1y'28, 1936 FOREIGN PATENTS Country Date France Feb. 5, 1929 

